This disclosure relates to prostheses and methods for total hip joint replacement. Specifically, this disclosure relates to prostheses that allow for high offset hip joint replacement and to instrumentation and methods for preparing the calcar region of the proximal femur that results in greater bone preservation. A high impact stem design is described that may result in greater bone preservation of the calcar region of the proximal femur, which results in better load sharing and greater overall capability. To ensure that the increase in bone preservation does not result in the limitation of the resulting range of motion, additional instrumentation and techniques are presented that allow the surgeon to create chamfer cuts on the calcar to minimize the potential for impingement.
Total hip replacement procedures seek to replace a hip joint that has deteriorated in its functionality, range of motion, weight bearing and most, if not all, other performance and lifestyle attributes. This degeneration is commonly found in patients that suffer from osteoarthritis, rheumatoid arthritis or osteonecrosis (avascular necrosis). Total hip replacement typically involves amputation of the femoral head, neck and a portion of the top of the femur in order to replace these structures with prosthetic components.
Individual skeletal development and postures vary from person to person. This is in part due to the three dimensional orientation of the hip socket relative to the proximal femur. The distance between the center of rotation of the femoral head and a reliable anatomical landmark, such as the lesser trochanter, may be described as the vertical offset of the head center from the lesser trochanter. This distance may be measured parallel to the femoral shaft axis, and is relevant to postoperative leg length. In the anterior-posterior view (AP view), the distance between the head center and the shaft axis may be described as the lateral offset of the shaft axis from the head center, or as the medial offset of the head center from the shaft axis. It is often referred to simply as “offset.” Lateral offset may be relevant to postoperative hip abductor function. Lateral offset is independent of the neck-shaft angle. However, lateral offset may be expressed in terms of the neck-shaft angle and the neck length, which is the distance along the neck axis between the head center and the shaft axis or some other reliable landmark. The neck-shaft angle varies through a range of angles, approximately 127-140 degrees for most people. The neck length varies as well. In the lateral view, the distance between the head center and the shaft axis may be described as the anteversion offset of the head center from the shaft axis, or simply anteversion, if the head center is anteriorly displaced from the shaft axis, or as retroversion if the head center is posterior to the shaft axis. Anteversion or retroversion may be relevant to postoperative range of motion.
Developed by Sir John Charnley in the 1960s, the original Charnley total hip arthoplasty was a stainless steel femoral prosthesis with a small collar, a rectangular cross section and a 22-mm femoral head. Subsequent designs evolved to include different head sizes (22, 25, 25.4 28, 32 and 35 mm), different femoral component lengths (ranging from 110 mm to 160 mm for standard prostheses), different cross sections (square, round, oval, I-beam) and a porous coating for bone ingrowth attachment and metal backing for the acetabulum (cemented or porous coated).
The neck-shaft angle and/or neck length of a prosthesis can also be highly varied in order to replicate natural anatomy or correct deformity. If the neck-shaft angle and/or neck length of a prosthesis are set so that the lateral offset is comparable to an average lateral offset value for intact normal femora, then this prosthesis may be said to have a conventional or standard offset. However, if the neck-shaft angle and/or neck length are set so that the lateral offset is relatively large, this is referred to as a high offset prosthesis. For example, the neck-shaft angle and/or neck length may be set so that the lateral offset is at least 10 mm greater than the average lateral offset value.
Any of the relevant dimensions of a prosthesis may be set so that they are comparable to an average value for intact normal femora. These dimensions may be said to be conventional dimensions. Likewise, any of the relevant dimensions may be set incrementally greater than the corresponding average value, in which case the prosthetic dimensions may be said to be augmented. It follows that smaller than average dimensions may also be selected.
A surgeon will typically measure both hip joints, including the neck-shaft angles, vertical offset, lateral offset, and leg length, prior to performing a total hip replacement procedure. These measurements allow the surgeon to match the replacement joint as closely as possible to the angles and dimensions of the original hip joint in order to achieve satisfactory range of motion, leg length, soft tissue tension, and stability. These measurements may also allow the surgeon to correct deformity or other conditions in and around the operated joint by matching the replacement joint to the contralateral hip joint.